Scanning electron microscope with means to remove low energy electrons from the primary electron beam

ABSTRACT

In scanning electron microscopes, the beam of primary electrons generated by an electron gun contains low energy stray electrons as it passes through the condenser lens system on route to the specimen. The stray electrons adversely affect the secondary electron image. By incorporating a retarding electrode maintained at negative potential as described in this specification, the undesirable stray electrons are removed from the beam of primary electrons on the basis of the energy difference between the two groups of electrons.

United States Patent Y Hashimoto et alt [111 3,792,263 [4 1 Feb. 12, 1974 SCANNING ELECTRON MICROSCOPE WITH MEANS TO REMOVE LOW ENERGY ELECTRONS FROM THE PRIMARY ELECTRON BEAM Inventors: Hiroshi Hashimoto; Susumu Takashima, both of Tokyo, Japan Assignee: Nihon De nshi Kabushiki Kaisha,

Akishi ma, Japan Filed: Sept. 13, 1972 Appl. No.: 288,688

US. Cl. 250/311, 250/399 Int. Cl. I-I0lj 37/26, 60111 23/22 Field of Search '250/495 A, 495 PE, 49.5 ED, 250/49.5 E

References Cited UNITED STATES PATENTS l2/l97l Grubic, .lr. 250/49.5

3,629,579 12/1971 Naitou ..250/49.5

Primary Examinerwilliam F. Lindquist Attorney, Agent, or FirmWebb, Burden, Robinson & Webb In scanning electron microscopes, the beam of primary electrons generated by anelectron gun contains low energy stray electrons as it passes through the condenser lens System ;on route to the specimen. The stray electrons adversely affect the secondary electron image. Byincorporating a retarding electrode maintained at negative potential as described in this specifi- ABSTRACT cation, the undesirable stray electrons are removed from the beam of primary electrons on the basis of the energy difference between the two groups I of electrons.

5 Claims, 5 Drawing Figures PAIENIED-FEB: 121974 3' 792 2 3 SHEET 1 0F 3 SCANNI NG GENERATOR SCANNING ELECTRON MICROSCOPE WITH MEANS TO REMOVE LOW ENERGY ELECTRONS FROM THE PRIMARY ELECTRON BEAM This invention relates in general to a scanning electron microscope which permits the observation of a secondary electron image. More specifically, however, it relates to a scanning electron microscope incorporating a means for removing undesirable stray electrons included in the electron beam irradiating a specimen.

The general principle of the scanning microscope is as follows: A finely focused beam of electrons impinges and scans the surface of a specimen causing the specimen to emit a flow of secondary electrons. These secondary electrons are then detected and converted into an electrical signal the strength of which is proportional to the rate at which the electrons are detected. This signal is then applied to the control grid of a cathode ray tube in order to modulate the brightness of the tube which scans in'synchronism with the primary electron beam. By so doing, a magnified image of the specimen is formed on the screen of the cathode ray tube.

In a conventional scanning electron microscope, however, the secondary electrons detected by the detector include not only the secondary electrons emitted from the specimen but also low energy stray electrons included in the primary electron beam as it passes through the condenser lens system. As a result, the background is increased at the center of the secondary electron image, and quantitative analysis of the secondary electrons emitted from the specimen becomes difficult.

Accordingly, an object of this invention is to remove the above-mentioned low energy stray electrons from the primary electron beam.

Briefly, according to this invention, a scanning electron microscope is provided with a retarding electrode maintained at negative potential or a deflecting means having some field strength in the vicinity of the lens field near the specimen so as to remove the stray electrons according to the energy difference between the primary electrons and the stray electrons.

Further. featuresand other objects and advantages will become apparent by a study of the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram showing the lens system of a conventional scanning electron microscope;

FIGS. 2 and 3 are diagrams showing embodiments of this invention incorporating a retarding electrode maintained at negative potential; and,

FIGS. 4 and 5 are diagrams showing embodiments of this invention incorporating a deflecting means having some field strength.

Referring to FIG. 1, there is shown a typical scanning electron microscope having an electron optical column I, an electron gun 2 for generating a primary electron beam 3 which passes through an anode 4, a first condenser lens 5, a first condenser lens aperture 6, scanning deflection coils 7x and 7y, a second (or final) condenser lens 8, and a second (or final) condenser lens aperture 9, finally impinging on a specimen l0 mounted on a specimen stage 11. Holders 12 and 13 are arranged for holding the aperture plates 6 and 9 in place. A DC. voltage source 14 is connected between the specimen and the entrance of the detector 15, the polarity of which is arranged so as to attract the secondary electrons emitted from the specimen into said de- SP l tector 15. In the detector, the electrons are converted into an electrical signal which is amplified by an amplifier 16 prior to being applied to the control grid of a cathode ray tube 17 as a brightness modulation signal. Finally, a scanning generator 18 supplies signal to scanning deflection coils 17): and 17y, forming part of the display means and to the scanning deflecting coils 7x and 7y, in order to display a secondary electron scanning image on the screen of the cathode ray tube.

Of the primary electrons generated by the electron gun, only a very few of them succeed in reaching the surface of the specimen. The majority of the electrons in fact strike up against the surface of the condenser lens aperture plates or against the inner wall of the lenses and are absorbed or scattered thereby. The scattered electrons rejoin the primary electron beam after losing a greater part of their energy (velocity) through repeated impingements. Furthermore, the impingement of the primary electrons produces many secondary electrons which also find their way into the primary electron beam. As a result, the beam is made up of high velocity primary electrons and low velocity scattered and secondary electrons 19, the low velocity electrons being hereinafter referred to collectively as stray electrons. Now, when the electron beam reaches the deflection coils, most of the stray electrons are prevented from passing through, as their angle of deflection is much larger than that of the primary electrons due to their energy difference. However, when the deflecting magnetic field or electric field is zero or thereabout; i.e., at the midway point of the scan area at which the beam strikes the specimen, most of the stray electrons pass through the coils and proceed toward the specimen. When they reach the axially symmetrical magnetic field produced by the second (or final) condenser lens, the radius of their spiral path becomes inversely proportional to the field intensity. So, as the lens field is strong enough to focusthe primary electron beam on the specimen, the radius of the spiral path of the stray electrons becomes very small enabling them to pass through the second (or final) condenser lens aperture and reach the vicinity of the specimen where they are drawn with the detector together with the secondary electrons emitted by the specimen. As a result, the image formed on the screen is not a pure secondary image but contains a background signal which makes the center of the image bright.

i FIG. 2 shows the second (or final) condenser lens land the specimen chamber of a scanning electron microscope according to this invention. In the figure, a means for removing the low energy stray electrons from the high energy primary electrons is disposed just above the lens aperture 9, said means being comprised of three non-magnetic plates 20, 21 and 22 and two layers of insulators 23 and 24. Plates 20and 22 are maintained at ground potential and retarding electrode plate 21 is maintained at a negative potential, for example,

at about -10 to -30 volts by means of a DC. voltage source 25. By so doing, the stray electrons included in the primary electron beam are reflected by the electric field formed above the aperture 9 and are thus prevented from passing through theaperture and reaching the detector. The primary electrons, on the other hand, due to their high velocity (energy) remainon course and impinge on the specimen surface as intended. Actually, plates 20 and .22 function as only shields and are not always necessary.

The embodiment shown in FIG. 3 differs slightly from that described above in that a retarding electrode 26 is attached under the pole piece 27 of the lens 8 by insulator 28 and is maintained at a negative potential by means of a DC. voltage source 29. By so doing, the stray electrons gathered along the optical axis near the lens field are reflected by the electric field formed above the retarding electrode 26.

FIG. 4 shows another embodiment incorporating an electrostatic deflecting means near the lens field so as to deflect the stray electrons gathered along the optical axis near the lens' field. The deflecting means is composed of deflecting electrodes 30 and 31, two insulators 32 and 33 and supporting plate 34. DC voltage source 35 keeps electrode 30 at a negative potential and electrode 31 at a positive potential. Accordingly, the stray electrons gathered along the optical axis near the strongmagnetic field are deflected and thus prevented from passing through the aperture.

FIG. 5,shows another embodiment according to this invention. This embodiment incorporates a magnetic deflecting means near the lens field. The deflecting means, in this case, is composed of coils 36 and 37 and a DC. current source 38 and supporting plate 39. The function of this deflecting means is the same that of the embodiment shown in FIG. 4.

Having thus described the invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.

We claim:

1. In a scanning electron microscope comprising means for creating an electron beam, means including condenser lenses for focusing said beam along an optical axis, means for causing said beam to scan over a specimen, means for detecting secondary electrons emitted from said specimen and means in synchronism with said scanning means for displaying in a raster a signal indicative -of the electrons striking the detecting means, the improvement comprising means disposed in the vicinity ofa final condenser lens near said specimen for providing a substantially unvarying electron diverting low strength field near the optical axis for removing the low energy electrons from the high energy primary electrons generated by the electron beam creating means.

2. The improvement to a scanning electron microscope according to claim 1 wherein said low energy electron removing means comprises a retarding electrode maintained at a negative potential.

3. The improvement to a scanning electron microscope according to claim 2, wherein said retarding electrode is maintained at about l0 to 30 volts.

4. The improvement to a scanning electron microscope according to claim 1, wherein said low energy electron removing means comprises an electrostatic deflection means.

5. The improvement to a scanning electron microscope according to claim 1 wherein said low energy electron removing means comprises a magnetic deflection means.

UNITED STATES PATENT OFFICE CERTIFICATE OF" CORRECTION 'Patent: No. 3 792, 263 Dated February 12, 1974 Inv n Hiroshi Hashimoto an d Susunou Takashi ma It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

- After the line listing the applicetion serial number insert the following:

Foreign Application Priority" Data septembef'l, 1971 Japan 46-84252 September 16, 1971 Japan 46-84253-- Signed and sealed this 24th day of September 1974.

(SEAL) Attesti MCCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. In a scanning electron microscope comprising means for creating an electron beam, means including condenser lenses for focusing said beam along an optical axis, means for causing said beam to scan over a specimen, means for detecting secondary electrons emitted from said specimen and means in synchronism with said scanning means for displaying in a raster a signal indicative of the electrons striking the detecting means, the improvement comprising means disposed in the vicinity of a final condenser lens near said specimen for providing a substantially unvarying electron diverting low strength field near the optical axis for removing the low energy electrons from the high energy primary electrons generated by the electron beam creating means.
 2. The improvement to a scanning electron microscope according to claim 1 wherein said low energy electron removing means comprises a retarding electrode maintained at a negative potential.
 3. The improvement to a scanning electron microscope according to claim 2, wherein said retarding electrode is maintained at about -10 to -30 volts.
 4. The improvement to a scanning electron microscope according to claim 1, wherein said low energy electron removing means comprises an electrostatic deflection means.
 5. The improvement to a scanning electron microscope according to claim 1 wherein said low energy electron removing means comprises a magnetic deflection means. 